Bronsted acidic sites

Factors other tlian tire Si/Al ratio are also important. The alkali-fonn of zeolites, for instance, is per se not susceptible to hydrolysis of tire Al-0 bond by steam or acid attack. The concurrent ion exchange for protons, however, creates Bronsted acid sites whose AlO tetraliedron can be hydrolysed (e.g. leading to complete dissolution of NaA zeolite in acidic aqueous solutions). 

Possible role of the induced acidity and basicity in catalysis and environmental chemistry is discussed. The suggested mechanism explains the earlier reported promotive effect of some gases in the reactions catalyzed by Bronsted acid sites. Interaction between the weakly adsorbed air pollutants could lead to the enhancement of their uptake by aerosol particles as compared with separate adsoi ption, thus favoring air purification. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Bronsted acid sites in HY-zeolites mainly originate from protons that neutralize the alumina tetrahedra. When HY-zeolite (X- and Y-zeolites... 

Reaction between a Bronsted acid site (H+) and an olefin... 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

Fig. 8). The Os result was explained on the basis of an interaction between the Bronsted acid sites and Fe + species. The same type of Interaction is believed to indirectly expand the s- ave function and to decrease the s-electron density at the 57pe nucleus, thus giving an Increase in 6 at the same "Si02/Al203 ratio" of 17. 

We have shown that the Fe or Fe-Co/Zeollte systems with a vs H/T type results provide a unique probe for obtaining particle sizes (3) of the metal clusters, and can be extended for computing a particle-size distribution. In addition, electronic interactions between the Bronsted acid sites and the Fe species can be elucidated. 

The infrared spectra were recorded after equilibrating the reduced and evacuated solids with an excess of pyridine vapor and further evacuation at various temperatures. After evacuation at 423 K there is no more physically adsorbed pyridine. There is no characteristic band of pyridine adsorbed on Bronsted acid sites (no appearance of the 19b vibration at 1540-45 cm" ) [11,12]. The OH groups observed on the solids are thus non acidic. The existence of Lewis acid centers (coordinatively unsatured Al " or Zr ) is proven by the presence of the 19b vibration at 1440-50 cm" and of the 8a vibration at 1610-1620 cm". The absorbances of the 1440-50 cm" band show that the acidity difference between the Pd/Al203 and PdyZr02 solids is not significant. 

Figure 3.53. IR transmission absorption spectrum of pyridine adsorbed on partly dehydroxylated HY zeolite (Van Bekkum et al, 1991) B = Bronsted acid sites L = Lewis acid sites.

Rising of CO partial pressure resulted in the appearance of a series of absorption bands with v (CO) 2176, 2156 and 2136 cm" in the area of CO stretching vibrations. The band at v (CO) 2176 cm can be attributed to the interaction of CO with weak Bronsted acid sites while the band at v (CO) 2156 cm can be attributed to the complex of CO with non-acid hydroxyl groups. The band with wavenumber v (CO) 2136 cm" can be related to physical-adsorbed CO over Silica. 

At an industrial scale, the esterification catalyst must fulfill several conditions that may not seem so important at lab-scale. This must be very active and selective as by-products are likely to render the process uneconomical, water-tolerant and stable at relatively high temperatures. In addition, it should be an inexpensive material that is readily available on an industrial scale. In a previous study we investigated metal oxides with strong Bronsted acid sites and high thermal stability. Based on the literature reviews and our previous experimental screening, we focus here on application of metal oxide catalysts based on Zr, Ti, and Sn. 

Two-Dimensional correlation analysis to study Bronsted acid sites in zeolites... 

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

The shift of the acidic OH-band due to the interaction of the Bronsted acid sites with hexane reactant is clearly accompanied by the appearance of the absorption bands in the vCh region (fig. 2). The Av0h, C6 values listed in table 1 suggest that the apparent acidity of the ZSM-5 and mordenite samples is distinctly different (113-116 and 85-94 cm 1 shifts, respectively) and significantly higher than that of the zeolite Y samples (41-52 cm"1). 

NH4 ion-exchange capacity, taken as the concentration of Bronsted acid sites... 

Apparent site-specific bimolecular rate constant, obtained by relating the kB app iimol-gcat."1 s"1) to the concentration of Bronsted acid sites (pmol-gc., 1). 

In spite of the nominal absence of Bronsted acid sites, it is possible to introduce copper species in the S-l matrix by following an ion-exchange procedure. The unit cell of the S-l can be written as follow [1] ... 

The aluminum is incorporated in a tetrahedral way into the mesoporous structure, given place to Bronsted acidic sites which are corroborated by FTIR using pyridine as probe molecule. The presence of aluminum reduces the quantity of amorphous carbon produced in the synthesis of carbon nanotubes which does not happen for mesoporous silica impregnated only with iron. It was observed a decrease in thermal stability of MWCNTs due to the presence of more metal particles which help to their earlier oxidation process. 

Figure 2 (Left) shows the 27Al NMR spectra for the aluminosilicates. All of them displayed a tetrahedral incorporation of aluminum inside the silica network. That is corroborated by the signal at 55 ppm [9, 10] which also become more intense with the decreasing of Si/Al ratio. Octahedral aluminum was observed just for the samples with the lowest Si/Al ratio. Tetrahedral aluminum gives place to strong Bronsted acid sites, which were identified by the interaction of these groups with pyridine that generates a...

The tetrahedral Al incorporated in mesoporous silica reduces considerably the quantity of amorphous carbon, increasing the MWCNTs selectivity, due to the formation of strong Bronsted acidic sites, which allow a better dispersion of iron metallic clusters. The Fe/Al-MCM41 (10) showed the best results in CNT purity and yield. This indicates that the aluminum content and its tetrahedral structural incorporation play an important role in the CNT syntheses. 

Figure 4 presents correlation between the basic and acid activities obtained with the model reaction and the surface area of 1230 cm 1 band of adsorbed CO2 species after evacuation at RT under vacuum (Figure 4a) and the quantity of Bronsted acid sites able to retain DMP at 150°C respectively (Figure 4b). 

Figure 4b shows a quasi linear relation between the activity in alcohol dehydration and the quantity of Bronsted acid sites obtained by DMP adsorption. However a low amount of sites is active in dehydration (on theta alumina) but practically no Bronsted acid sites are detected by DMP adsorption because of their low number and strength. 

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